1. Field of the Invention
The present invention relates to an ink jet printing apparatus that prints a print medium by moving a print head ejecting ink, in a direction crossing a direction in which the print medium is conveyed, and in particular, to an ink jet printing apparatus that can adjust deviation of landing positions of ink droplets if a printing operation is performed by reciprocating the print head, as well as a printing control method for the ink jet printing apparatus.
2. Description of the Related Art
In the prior art, serial printer type ink jet printing apparatuses perform a printing operation while moving a print head in a direction crossing a direction in which the print medium is conveyed. With such an inkjet printing apparatus, a position at which ink lands on a print medium may vary between a forward scan and a backward scan of the print head during a printing operation. To solve this problem, what is called registration is carried out to, for example, adjust ejection timing.
Ink jet printing apparatuses having such a registration function are widely applied to PC printers, facsimile machines, multifunction printers, and the like.
Description will be given of an example of registration conventionally carried out in ink jet printing apparatuses capable of bidirectional printing.
First, data required to print the test patterns shown in FIG. 7C is divided. The test pattern shown in FIG. 7A is printed during forward printing. The test pattern shown in FIG. 7B is printed during backward printing. In both cases, the patterns are formed in the same area. Then, the print medium is conveyed by a predetermined amount, and similar test patterns are printed in an area that has not been printed yet. This process is repeated a number of times. However, for each printing area, timing for ink ejection is varied during at least one of forward scan printing and backward scan printing. This provides output results such as those shown in FIGS. 8A to 8E. Then, one of these results is selected which is most similar to the test pattern shown in FIG. 7C. Specifically, the pattern shown in FIG. 8C is selected to determine ink ejection timing for the smallest variation in ink droplet landing positions between forward printing and backward printing. That is, a registration value is determined. If a plurality of print element arrays are formed in the print head, each print element array is registered.
To accurately and stably achieve registration such as that described above, it is necessary to print the test patterns so as to maintain a fixed distance between a print element arranged surface of a print head 13 and a sheet surface (this distance will hereinafter referred to as a “sheet distance”) Thus, in the prior art, the test patterns are printed by sandwiching the sheet between a conveying roller 8 and a pinch roller 12 and pressing these rollers against the top surface of a platen 6.
Registration values thus obtained are used from the start to end of a printing operation as shown at step S20 in the flow chart shown in FIG. 9.
On the other hand, ink jet printing apparatuses have recently commonly been used as equipment that outputs images photographed using a digital camera. The outputted images are desired to achieve photograph-like high-grade printing all over print media as in the case of marginless printing (hereinafter referred to as a “full bleed printing”).
However, with the prior art registration, after the trailing edge (hereinafter referred to as a “back end of sheet”) of a print medium has passed through the conveying roller, an urging force exerted on the print medium decreases significantly. Thus, the sheet distance changes from the one obtained by the registration to shift the landing positions of ink droplets (misregistration) As a result, after the trailing edge of the print medium has passed through the conveying roller, the printed image is significantly degraded compared to the image obtained before the passage of the trailing end.